The upper layer of earth in which plants grow, a black or dark brown material typically consisting of a mixture of organic remains, clay, and rock particles.
The Breakdown of Soil
- Organic matter breakdown is not a single chemical transformation but a complex process. Breakdown of organic matter involves chemical alteration of organic matter, physical fragmentation and finally release of mineral nutrients. Organic matter breakdown is a biological process because it it the soil organisms ( earthworms, ants, beetles ) that bring about the chemical and physical changes. Different organisms are involved in different stages of these processes.
- Breakdown starts almost immediately after the organism, or part of it, dies. The organic matter is colonised by microorganisms that use enzymes to oxidise the organic matter to obtain energy. For leaves and roots their surfaces are colonised by microorganisms even before they die. Soil animals such as earthworms assist in the decomposition of organic matter.
- During decomposition the organic molecules in organic matter are broken down into simple organic molecules that require further decomposition or into mineralised nutrients. the compounds in organic matter vary in the ease with which microorganisms can break them down. the first organic compounds to be broken down are those that are easy to break down, including amino acids and sugars. Cellulose will break down more slowly and lignin, phenols and waxes will remain in the soil for the longest time.
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